1. Field of the Invention
The present invention relates to an optical pickup actuator in an optical disc player. More particularly, the present invention relates to an optical pickup actuator for driving an objective lens included in an optical pickup device adapted for recording and/or reproducing the data signals on or from an optical recording medium such as an optical disc.
2. Description of the Prior Art
There are apparatuses which record the data signals or read the recorded data signals on or from the signal recording medium using an optical pickup device. The compact disc (CD) player is one of these types of apparatus. The CD player reproduces the digital signals formed on the signal recording surfaces of the compact discs which have projections called pits. The CD player includes an optical pickup device which is installed just below an optical recording medium such as an optical disc.
Normally an optical pickup device includes an objective lens holder having an objective lens retained therein. Tracking coils and a focusing coil are disposed on the outer periphery of the lens holder. Control signals are supplied to the tracking coils and the focusing coil in order to perform tracking control and focusing control operations. Thus, the optical pickup device is required to support the objective lens holder so that the objective lens may freely move in the tracking direction or the focusing direction in accordance with the input control signals.
The optical pickup device requires a focusing control signal to control the optical beam in the optical axis direction, the so-called focusing direction, so that the optical beam may be correctly converged on the signal recording surface of the optical disc. The laser beam is produced from the optical pickup device. Also, it is necessary to perform a tracking control operation for controlling the laser beam in the radial direction of the optical disc, the so-called tracking direction, to cause the laser beam to follow the signal track on the optical disc. This type of operation is called linear tracking method.
When the optical pickup device has sought a desired track by the track tracking operation, the optical pickup device performs a track following operation.
In the track following operation, the object lens of the optical pickup device focuses the laser beam on the track of the optical disc which is being followed. When the laser beam is not accurately focused on the track of the optical disk being followed or when a focus of the laser beam is slightly deviated from the track, the optical pickup actuator installed in the optical pickup device performs the focusing operation or the tracking operation.
With these focusing and tracking operations of the actuator, the objective lens accurately follows the track while focusing the laser beam on a desired track. Accordingly, the optical pickup actuator in the optical disc player records the data signals on the optical disc or reproduces the digital signals from the optical disc with the aforementioned linear tracking method.
FIG. 1 is a perspective view for showing a structure of an example of a conventional linear tracking type optical pickup actuator.
As shown in FIG. 1, a conventional optical pickup actuator 10 includes a base plate 38 having a laser beam through hole (not shown) at a central portion thereof through which a laser beam passes, a pair of facing outer yokes 12 and a pair of facing inner yokes 14 each formed at left and right sides on base plate 38 while centering the laser beam through hole. A damping-gel plate 26 is secured to an outer surface of one of outer yokes 12.
An objective lens holder 40 is disposed above the laser beam through hole and provided with an object lens 18. Objective lens holder 40 has a focusing coil 24 which is installed on front, rear, left and right outer surfaces for the focusing operation and first, second, third and fourth tracking coils 73, 74, 75 and 76 each pair of which is attached to left and right outer surfaces of focusing coil 24.
A pair of magnets 16 is respectively disposed between outer yokes 12 and inner yokes 14 for driving objective lens 18 in the focusing operation direction shown by an arrow f and the tracking operation direction shown by an arrow t by forming magnetic flux with outer yokes 12 and inner yokes 14 toward focusing coil 24 and first, second, third and fourth tracking coils 73, 74, 75 and 76.
Objective lens holder 40 has a plurality of projections 44 formed symmetrically on front and rear outer surfaces in which suspensions 30 are fixed. Both ends of suspensions 30 are respectively fixed to projections 44 and damping-gel plate 26 to eliminate undesired vibrations caused by vibrations of objective lens 18 in the optical-axis direction or in the radial direction and external shocks.
In the conventional optical pickup actuator in an optical disc player having the above structure, a pair of outer yokes 12 and a pair of inner yokes 14 are installed at left and right sides of base plate 38 in order to be elements constituting a closed magnetic circuit. In addition, the volumes of damping-gel plate 26 and printed-circuit board 28 are large. Thus, the overall height and length of the optical pickup actuator are increased, and the steps of assembly process become complicated with an increasing number of parts. As a result, workability and productivity decrease.
As an example of the aforementioned optical pickup actuator, U.S. Pat. No. 5,144,607 (granted to Tomoyuki Ishida et al.) discloses a structure of an optical pickup actuator adapted for the above-mentioned linear tracking operation. This optical pickup actuator includes an objective lens bobbin (or holder) provided with an objective lens, an objective lens supporting member for movably supporting the objective lens bobbin in the focusing and the tracking operation directions, focusing coils and tracking coils inserted within the objective lens bobbin, and a magnetic yoke for driving the objective lens in the focusing and tracking operation directions by forming magnetic flux toward focusing coils and tracking coils. In the above conventional optical pickup actuator, focusing coils and tracking coils are inserted in two coil housings of the objective lens bobbin. The objective lens supporting member includes first and second objective lens supporting members arranged at front and rear outer surfaces of the lens bobbin. Each of the first and the second objective lens supporting members has a pair of suspension plates and a pair of connecting plates interconnected to respective ends of a pair of suspension plates to each other, thereby forming a rectangular frame. A supporting member is inserted between one side of facing connecting plates of the first and the second objective lens supporting members. Further, the magnetic yoke includes a bottom plate, first and second side plates, a connecting plate, and first and second lugs, thereby forming a rectangular frame.
The above-mentioned optical pickup actuator of an optical pickup device includes the objective lens supporting member for supporting the objective lens in such a manner to eliminate undesired resonant vibrations during the operation of the optical pickup actuator. The optical pickup actuator also includes the magnetic yokes constituting the magnetic circuit section and having the structure for eliminating vibrations of the magnetic yokes without using a fastener for suppressing the vibrations.
However, since the optical pickup actuator requires assembled structures provided on both sides of the objective lens bobbin, the structure of the optical pickup device becomes complicated as described above, and the overall height and length of the optical pickup actuator increase.
The aforementioned optical pickup actuator 10 has the structure in which U-shaped inner yoke 14 is mounted on base plate 38. A conventional optical pickup actuator having a different structure from that of optical pickup actuator 10, has a pair of inner yokes formed by bending a pair of inner quadrilaterals upwards at right angles after cutting a base plate with a flat plate shape along outlines of the inner quadrilaterals except bending portions corresponding to parallel outer sides of the inner quadrilaterals with respect to an axis of symmetry drawn between the inner quadrilaterals which are included at a central portion of the base plate and disposed facing each other. Also, this optical pickup actuator has a pair of outer yokes formed by bending a pair of outer quadrilaterals upwards at right angles after cutting the base plate along outlines of the outer quadrilaterals except bending portions corresponding to parallel outer sides of outer quadrilaterals with respect to an axis of symmetry drawn between the outer quadrilaterals which are included in left and right portions of the base plate and aligned with the inner quadrilaterals. Since a pair of outer yokes and a pair of inner yokes are respectively disposed in left and right portions and at the central portion of the base plate, sizes of the inner yokes are restricted in a step for cutting outlines of the quadrilaterals. As a result, heights of the inner yokes are lower than those of the outer yokes. A pair of magnets is secured to the inner lateral sides of a pair of outer yokes, so that magnetic lines of forces are directed from the magnets to the inner yokes. Left and right portions of a focusing coil and first, second, third and fourth tracking coils are disposed between a pair of magnets and a pair of inner yokes.
Since the heights of the inner yokes are lower than those of the outer yokes, in case lengths of the magnets are longer than those of the inner yokes, the absolute values of magnetic flux densities become small, and intervals between the magnetic lines of forces are becoming narrower while approaching the inner yokes. Thus, distributions of the magnetic fluxes with respect to the focusing coil and the first, second, third and fourth tracking coils, are not uniform.